This invention relates to a method and apparatus for separating plasma into a high molecular weight plasma component and a low molecular weight plasma component utilizing porous hollow fiber on thin channels having membrane walls.
Presently, liquids can be separated into a high molecular weight component and a lower molecular weight component by being passed along thin channels having membrane walls or porous hollow fibers when the membranes of the hollow fibers have a pore size which exclude the high molecular weight component while permitting passage therethrough of the low molecular weight component. In human blood plasma, the separation of the mixture by ultrafiltration is limited by the layer of the high molecular weight component concentrated on the membrane or fiber surface as a result of concentration polarization. Membranes are selected which allow substantial permeation of the albumin portion of the plasma while simultaneously achieving substantial rejection of globulins. It has often been observed that many membranes have these properties when tested with albumin alone or globulin alone. However, in the vast majority of cases, when the albumin and globulins are mixed together, the permeation of albumin is reduced while the globulins continue to be substantially rejected. This undesirable effect becomes more pronounced the higher the flux and/or the higher the concentration of globulins in the mixture. For this reason, ultrafiltration has been used almost exclusively as a concentration tool, since systems that reliably separate species based on the molecular size have not been possible especially when the species are of similar molecular size as in the case of albumin and globulins.
To circumvent the effects of concentration polarization, several processes have been developed which modify the feed plasma source to improve liquid flux and selectivity. For example, Ohno et al in U.K. patent application No. 2,065,129 discloses a separation process in which serum is diluted to reduce total protein and salt concentration while the pH is adjusted to between 3.8 and 4.7 prior to ultrafiltration.
Baeyer et al in the Journal of Membrane Science, 22 (1985) 297-315, discloses a process in which the plasma is first diluted by a factor of 12 prior to ultrafiltration. Malchesky et al in U.S. Pat. No. 4,350,156 discloses a process for removing macromolecules from plasma by cooling the plasma to about 10.degree. C. and then filtering the macromolecules from the cooled plasma to form a filtered low molecular weight plasma stream. This process does not utilize an ultrafiltration membrane.
It would be desirable to provide an ultrafiltration process utilizing open thin channels which effects passage of a lower molecular weight species through an ultrafiltration surface from a mixture of high molecular weight species and low molecular weight species at a level approaching that achieved with solutions containing a single low molecular weight species. In the case of plasma treatment wherein albumin is separated and recycled to a patient, it would be desirable to attain the highest albumin permeate flow rate through the separation membrane with a minimum extracorporeal volume (membrane area).